There has been known an NOx selective reduction catalyst (hereinafter, also referred to simply as an “NOx catalyst”) which purifies (removes or reduces) NOx contained in an exhaust gas from an internal combustion engine by using ammonia as a reducing agent. Ammonia or a precursor (e.g., urea) of ammonia is supplied into the exhaust gas upstream of the NOx catalyst. In addition, when determining whether removal or reduction of NOx in the NOx catalyst is carried out normally, or when determining whether the NOx catalyst is normal, there may be used an output value of an NOx sensor that is arranged at a location downstream of the NOx catalyst.
Here, it is known that in cases where the output value of the NOx sensor arranged downstream of the NOx catalyst has a decreasing tendency, in spite of the fact that the amount of NOx flowing into the NOx catalyst has increased, it is judged that there occurs an ammonia slip in which ammonia flows out from the NOx catalyst, and that the amount of supply of ammonia is excessive (for example, refer to a first patent literature).
In this connection, note that the NOx sensor detects ammonia as well as NOx, so the output value of the NOx sensor becomes a value corresponding to an NOx concentration and an ammonia concentration in the exhaust gas. Then, it is also known that when a gas concentration is equal to or greater than a threshold value for starting a diagnosis, and when the internal combustion engine is in a state where fuel is not injected, the pressure in an exhaust passage is made to go up, and in cases where a subsequent gas concentration is equal to or greater than a predetermined value, a determination is made that the NOx sensor is abnormal (for example, refer to a second patent literature).
In addition, it is also known that rationality or normality of an NOx sensor is decided by determining whether a change over time of an output value of the NOx sensor has changed in correspondence to a predetermined amount of ammonia having flowed out to a downstream side of a reduction catalyst (for example, refer to a third patent literature).